Currently, radio waves are widely used in various fields, and, particularly, great needs for a wireless broadband system which can be used regardless of places or time are expected in the future. In order to satisfy these needs, a new frequency is required to be secured, but frequency bands of 6 GHz or lower which are convenient to be used in a wireless broadband have already been used in a scrupulous manner. For this reason, in the future, even in a frequency which has already been allocated, a frequency band (a so-called white space) which can be used temporally, spatially, and technically is required to be flexibly used after sufficiently avoiding interference with an existing system.
Following this trend, research and development or standardization of a wireless communication system using a white space have progressed in countries all over the world. In Japan, the Ministry of Internal Affairs and Communications has made specific examination of institutional, technical issues or the like for directivity of the use of new radio waves or realization methods thereof, in “Study Group on the Use of New Radio Waves”. The report collected in July, 2010 proposed that “research and development should be promoted for developing technique which enables a plurality of frequency bands to be dynamically used and establishes prevention of interference with an existing system or the like, such as a spectrum sensing technique or a dynamic access technique, and effective use of radio waves should be required”, and twenty-five white space special districts were selected in April, 2011 in order to promote systemization and business development of services or systems using the white space, and research and development or demonstration tests thereof have progressed. On the other hand, outside Japan, in U.S.A, a commission for recognizing the use of a white space was adopted in November, 2008, and the Federal Communications Commission (FCC) has performed work including establishment of specific technical criteria or building of databases. As mentioned above, many countries are also performing research and development regarding a new wireless technique aiming at the use of a white space. In addition, standardization organizations such as IEEE SCC41, IEEE802.22, and IEEE802.19 are making frantic efforts to standardize a technique which highly uses a white space.
As described above, the white space has attracted much attention, and, above all, IEEE802.22 as a standardization specification of a wireless regional area network (WRAN) which realizes a wireless broadband by using a TV band frequency is considerably expected (NPL 1). In the TV band, channels are disposed with a margin for each region in order to prevent interference between channels, and thus there are many frequency channels (white spaces) which are not actually used. In addition, there is the time at which even an allocated frequency channel is not actually used due to termination of broadcasting in the middle of the night. Since unused frequency channels or unused time zones are predefined, this information is stored in a database for each place, and thus it is possible to relatively easily prevent interference with an existing system.
In order to avoid interference with the TV band frequency, an IEEE802.22 system has not only a database function which is the above-described database, but also functions such as a geolocation function, a spectrum sensing function, a spectrum manager function, and a self-coexistence function.
The spectrum manager function is implemented in a base station, and realizes determination of a frequency channel to be used according to a channel allocation algorithm called an etiquette mode so that frequency channels do not overlap each other between adjacent cells, that is, interfere with each other, on the basis of base station position information, frequency information obtained by the database function, policy, or the like. In addition, in this function, in a case where the use of an existing system such as terrestrial digital broadcasting in a base station or a terminal is detected by the spectrum sensing function, a structure (incumbent detection recovery protocol: IDRP) is provided in which transfer to another backup channel which is predefined. In the spectrum manager function, control is performed so that frequency channels do not overlap each other between adjacent cells as much as possible, but, in a case where the number of frequency channels is not sufficient, and a frequency channel has to be shared between base stations installed at a distance of giving interference to each other, the self-coexistence function is used. The self-coexistence function is a function which is necessary to share the same frequency channel without giving interference to each other even between adjacent cells in the IEEE802.22 system.
FIG. 1 is a flowchart illustrating the self-coexistence function of the 802.22 system.
If a base station (BS) is powered on (101), the BS acquires adjacent cell information such as a used frequency channel (102). The BS tries to acquire a used channel according to the etiquette mode (103 and 104), and if there is a usable channel, communication is performed in a normal mode by using the channel (105). If there is no usable channel, the BS enters a self-coexistence mode, and also uses a channel which is used in an adjacent cell. In the self-coexistence mode, cases are sorted according to whether interference occurring when a frequency is used along with the adjacent cell is given only between base stations or is also given to a CPE (terminal) belonging to an adjacent BS (107).
FIG. 2 illustrates a summary of the case sorting in the self-coexistence mode. An aspect of the above-described case sorting is illustrated in FIG. 2. In FIG. 2, ellipses 206 and 207) respectively indicate ranges in which a BS1 (203) and a BS2 (204) give interference. The BS1 (203) and the BS2 (204) are installed at positions where interference is given. As indicated by (202) of FIG. 2(b), if interference is given not only between the BSs but also to the CPE, the flow proceeds to on-demand frame contention (ODFC) (108), and, as indicated by (201) of FIG. 2(a), if interference is given only between the BSs, the flow proceeds to DS/US split adjustment (DUSA) (111).
Next, a description will be made of the OFDC and DUSA.
FIG. 3 illustrates a frame configuration of the 802.22 system. In the figure, a solid line indicates an allocated frequency channel. In addition, a transverse axis indicates time. FIG. 3 illustrates a situation of a normal mode (301) in which frequency channels (Channel1 to Channel3) are exclusively allocated to three adjacent BSs (BS1 to BS3), and the ODFC (302) in which a single frequency channel (Channel1) is shared therebetween. In the normal mode, all sixteen frames included in a super frame can be used in a frequency channel given to each base station (301), but, in the ODFC, sixteen frames of the shared Channel1 are shared by the respective BSs, and are used so that communication time does not overlap (302, 109). NPL 1 is referred to regarding details of a frame contention method between respective base stations or a frequency allocation method in the etiquette mode in the ODFC.
On the other hand, in the DUSA, not control is performed so that communication time does not overlap, but a split position of a downstream (DS) and an upstream (US) is used in common between a plurality of base stations sharing a frequency channel, and thus the base stations do not give interference to each other.
FIG. 4 is a schematic diagram of the DUSA, and FIG. 5 illustrates a frame configuration example in this case. In FIG. 4, ellipses (204, 205) indicate ranges in which each BS gives interference. It is assumed that a BS1 (203) and a BS2 (204) are installed at positions where interference is given, and a CPE (205) performs communication with the BS1 (203) but is located at a position where interference is not given from the BS2 (204). In the IEEE802.22 system, as illustrated in FIG. 5, a single frame is formed by a downstream (DS) subframe and an upstream (US) subframe. In the normal mode, a split position (a position where a ratio of the DS and the US is determined) of the DS and the US is independently determined for each cell according to a request from a base station or a terminal in each cell. (401) of FIG. 4(a) indicates a state in which interference is given due to transmission (DS) from the BS2 (204) when the CPE (205) performs transmission (US) toward the BS1 (203). As in an example illustrated in FIG. 5(a) (501), in a frame configuration in this case, DS amounts and US amounts are different in the respective BSs, that is, split positions of DS/US are different from each other, and the DS of the BS2 (204) having a larger DS amount given interference to the US of the BS1 (203). On the other hand, FIG. 4 (b) (401) illustrates a state in which the CPE (205) does not receive interference from the BS2 (204) even if the BS2 (204) performs transmission (DS) when the BS1 (203) performs transmission (DS) toward the CPE (205) (this is because it is assumed that the CPE (205) is located at a position where interference from the BS2 (204) is not given thereto). As illustrated in FIG. 5(b) (502), if split positions of DS/US are the same in the respective BSs, there is no interference unlike FIG. 5(a) (501). In other words, if the split positions of DS/US of the respective BSs are used in common, even when frames are simultaneously used between adjacent cells, a frequency channel can be shared without giving interference. In the DUSA of IEEE802.22, all cells sharing a frequency are synchronized with a split position where a DS ratio is highest, and thus a frequency channel is shared without mutual interference. PTL 1 discloses a method of minimizing interference which is given to the US of the CPE (205) belonging to the BS1 (203) illustrated in FIGS. 4(a) and 5(a) by the DS from the BS2 (204) sharing a frequency.
In a case where there is a request for frequency sharing from a new BS (106), channel allocation is performed again in the etiquette mode after including the new BS (103). In addition, in a case where there is a new request from the internal BS or CPE during the ODFC mode (110), the flow returns again to the channel allocation (103) in the etiquette mode.